Abstract: Reagents and method for genotyping mice and other animals by in situ Polymerase Chain Reaction amplification of a target polynucleotide in the tissue biopsy. The reagent is comprised of non-ionic detergents, a protease, a buffering agent, a metal ion cofactor, a chelating agent and a salt. The method is comprised of taking a tissue biopsy; admixing it with the reagent; a Lysing Cycle, an inactivation cycle, an amplification step and a detection step.